1. Field of the Invention
The invention relates to two component acrylic reactive adhesives used for joining plastic substrates or the like, and more specifically, relates to a method and apparatus for extending the stability and clear neutral color of organic peroxides by preventing the degrading and discoloring of such peroxides when used in two component acrylic reactive adhesives.
2. Description of the Prior Art
In the past acrylic bonding materials have been widely used to join filled acrylic sheets, polyester sheets and the like. Such materials are commonplace in private homes or industrial settings as countertops for kitchens and bathroom surfaces. Products utilizing two component acrylic adhesives for bonding sheet materials are produced under the trademarks CORIAN, SURELL, AVONITE, and FOUNTAINHEAD.
The aforementioned countertop materials are in high demand because of their functionality and aesthetic qualities. The aesthetic appeal is directly related to the success in the process of joining sheets or panels in such a way that the bonded area or seam is relatively inconspicuous. The seamless appearance of the countertops is highly desirable for both private homes and industrial sites. The appearance of a continuous, seamless surface requires a bonding adhesive that exactly matches the countertop material. Desirably the acrylic adhesives used to seam the solid plastic sheets together are prepared with pigmentation to exactly match the color of the sheets being seamed. However, it has been found that a problem exists with the organic peroxide solutions which initiate the reaction of the two component adhesives. The peroxide solutions turn yellow in color as a function of time and temperature. This yellow color in turn creates color matching problems between the seam and the plastic sheet surfaces resulting in conspicuous seams which are undesirable. With the advent of lighter and brighter surfaces being introduced due to consumer demands, these surfaces are even more sensitive to the yellowed initiators in the organic peroxide solutions.
Generally speaking, two component acrylic reactive adhesive is formed by mixing a first component including resins into a second component housing an initiator. The mixing process takes place within a few minutes. The mixed adhesive immediately begins to polymerize and eventually solidifies, in most instances, within one hour.
Normally the first component is formulated with an amine promoter which facilitates the timely polymerization of the adhesive. The promoter acts as an active source for the conversion of monomers to polymers. Preferably the second component is housed in a larger container than the first component so that the first can be added to the second. Usually the second component uses a reaction initiator in the form of organic peroxides in a solution. The reaction is initiated by the peroxides because they are highly unstable compounds which readily donate free radicals, or charged hydrogen and oxygen ions to the reaction mixture. Thus, the polymerization process when the organic peroxide is added to the polymer blend. These free radicals react with the monomers thus enhancing the reactivity of the monomers and speeding up the process of monomer linking to monomer to form long chains or polymers.
With all the advantages of these superior bonding adhesives, a basic problem still exists due to the uncontrollable degradation of the organic peroxides such that the solution turns from a clear organic liquid to a yellow liquid. The resulting yellow color makes it difficult if not impossible to match the color of bonding material to substrate.
The color change is due to the inherent instability of the free radical source in the organic peroxides and is a function of time and temperature. Thus, the change from clear to yellow increases over longer periods of time and/or higher temperatures. Also light may accelerate the degradation of the organic peroxide causing the color to shift from clear to yellow due to destabilization of the peroxide.